Which one of the following salts is most acidic in water?

For a weak acid with dissociation constant $K_a$ and its conjugate salt with hydrolysis constant $K_h$,which of the following represents their relationship?

What happens when solid $Na_2CO_3$ is dissolved in water?

If the $pH$ of a solution is less than $7$,it will be ......

Explain the following with reasons:
$(i)$ $NaCl$ solution is neutral. The solution of strong acid and strong base salts is neutral.
$(ii)$ $NaCH_3COO$ solution is basic. The solution of strong base and weak acid salt is basic.
$(iii)$ $NH_4Cl$ solution is acidic. The solution of weak base and strong acid salt is acidic.
$(iv)$ The solution of $NH_4CH_3COO$ is (almost) neutral. The solution of weak acid - weak base salts is almost neutral.

If the $K_a$ for cyanoacetic acid is $3.5 \times 10^{-3}$,what is the degree of hydrolysis for a $0.05 \ M$ solution of sodium cyanoacetate?